1. Field of the Invention
The present invention relates to a high-frequency low-pass filter and more particularly to a high-frequency low-pass filter having a strip line electrode for use as an inductor.
2. Description of the Prior Art
FIG. 15 is a perspective view showing an example of a conventional high-frequency low-pass filter. The high-frequency low-pass filter 1 shown in FIG. 15 includes a dielectric substrate 2 having first and second main surfaces. On the entire surface of the first main surface of the dielectric substrate 2, an earth electrode 3 is formed. In the center of the second main surface of the dielectric substrate 2, two microstrip line electrodes 4a and 4b forming first and second inductors are located. Furthermore, on the second main surface of the dielectric substrate 2, a first capacitive open-circuited stub electrode 5a forming a part of a first capacitor and an input electrode 6a forming an input terminal extend from end of one microstrip line electrode 4a. A second capacitive open-circuited stub electrode 5b forming part of a second capacitor extends from the other end of microstrip line electrode 4a and one end of the other microstrip line electrode 4b. A third capacitive open-circuited stub electrode 5c forming part of a third capacitor and an output electrode 6b as an output terminal extend from the other end of the other microstrip line electrode 4b.
FIG. 16 is a perspective view showing another example of a conventional high-frequency low-pass filter. Compared with the high-frequency low-pass filter shown in FIG. 15, in the high-frequency low-pass filter 1 shown in FIG. 16, three chip capacitors 7a, 7b and 7c instead of three capacitive open-circuited stub electrodes are used.
The high-frequency low-pass filters 1 shown in FIG. 15 and FIG. 16 have an equivalent circuit shown in FIG. 17 in a form of a concentrated constant. That is, the high-frequency low-pass filters 1 shown in FIG. 15 and FIG. 16 have an input terminal IN and an output terminal OUT. Between the input terminal IN and the output terminal OUT, the first and the second inductors L.sub.1 and L.sub.2 are connected in series. Furthermore, the input terminal IN is grounded through the first capacitor C.sub.1, the connecting point between the first and the second inductors L.sub.1 and L.sub.2 is grounded through the second capacitor C.sub.2, and the output terminal OUT is grounded through the third capacitor C.sub.3.
In the conventional examples shown in FIG. 15 and FIG. 16, when a stray capacitance between the earth electrode and the microstrip line electrode is increased, an inductive impedance between both ends of the microstrip line electrode is decreased. Consequently, it is difficult to miniaturize and adapt in a lower frequency.
Furthermore, in the conventional examples shown in FIG. 15 and FIG. 16, when the stray capacitance between the earth electrode and the microstrip line electrode is increased, a frequency by which the impedance between both ends of the microstrip line electrode turns into a capacitive impedance is decreased. Consequently, it is difficult to adapt in a higher frequency.
Also, in the conventional examples shown in FIG. 15 and FIG. 16, an unnecessary passband is generated by resonance in the frequency of the wavelength ##EQU1## wherein L is the line length of the microstrip line electrode, .epsilon..sub.r is the relative dielectric constant around the microstrip line electrode, and N is an integral number. Therefore, a good spurious characteristic is not obtained.